At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available to use wireless transfer of data and more applications became available that operate based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users.
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radio communications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
The LTE wireless communication systems would support multiple-input-multiple-output (MIMO) antenna array configurations. The use of multiple antennas at the transmitter and/or the receiver side can significantly boost the performance of a wireless system. Such MIMO arrays of antennas have the potential of both improving data rates as well as increasing the diversity. The antennas in a MIMO configuration can be located relatively far from each other, typically implying a relatively low mutual correlation. Alternatively, the antennas can be located relatively close to each other, typically implying a high mutual correlation. Which correlation is desirable depends on what is to be achieved with the multi-antenna configuration, i.e. transmit diversity or spatial multiplexing. These characteristics of a MIMO communication system are discussed next.
Transmit diversity may be achieved by using multiple antennas at the transmitter side. The use of multiple transmit antennas provides an opportunity for diversity without the need for additional receive antennas and corresponding additional receiver chains at a user terminal. Transmit-antenna diversity, simply referred herein as diversity, is achieved by providing a large distance between the antennas at the transmitter. Different approaches can be pursued to realize the diversity. One such approach, the delay diversity, is illustrated in FIG. 1. In FIG. 1 a signal S is transmitted from a transmitter 10 to a receiver 12, at a first time, via a first antenna 14, and at a second time, later than the first time, via a second antenna 16 of the transmitter, to create artificial time dispersion. A time delay unit 18 introduces a controlled time delay. The delay diversity is “invisible” to the user terminal that includes the receiver 12. The user terminal will simply see a single radio-channel subject to additional time dispersion.
Another approach to achieve diversity is cyclic-delay diversity (CDD). This delay is similar to the delay diversity discussed with regard to FIG. 1 with the difference that cyclic-delay diversity operates block-wise and applies cyclic shifts rather than linear delays as shown for example in FIG. 2. In case of OFDM transmission, as used for example by LTE systems, a cyclic shift of the time-domain signal corresponds to a frequency-dependent phase shift before OFDM modulation, as illustrated for example in FIG. 3. Similar to delay diversity, this delay creates artificial frequency selectivity as seen by the receiver.
Still another approach to achieve transmit diversity is space-time coding. Space-time coding is a general term used to indicate multi-antenna transmission schemes where modulation symbols are mapped in the time and spatial domain to capture the diversity offered by the multiple transmit antenna. As shown in FIG. 4, a space-time transmit diversity (STTD) unit 19 operates on pairs of modulation symbols. The modulation symbols s0, s1, s2, and s3 are directly transmitted to the first antenna 14 while at the second antenna 16, the order of the modulation symbols within a pair is reversed, in addition to some sign changes and complex conjugate operations, as shown in FIG. 4. Optionally, the modulation symbols are sign-reversed and complex-conjugated as also shown in FIG. 4. An approach similar to the space-time coding is the space-frequency block coding (SFBC). The difference is that the encoding in SFBC is carried out in the antenna/frequency domain rather than in the antenna/time domain. Thus, space-frequency coding is easily applicable to OFDM and other “frequency-domain” transmission schemes. As shown in FIG. 5, the space-frequency transmit diversity (SFTD) maps a block of modulation symbols a0, a1, a2, a3, . . . to OFDM carriers of the first antenna 14, while the block of symbols −a1*, a0*, −a3*, a2*, . . . is mapped to corresponding subcarriers of the second antenna 16.
Spatial multiplexing is another characteristic of MIMO systems. Spatial multiplexing is achieved by using multiple antennas at both the transmitter and receiver as will be discussed next. The simultaneous availability of multiple antennas at the transmitter and the receiver can be used to create multiple parallel “data pipes” over the radio interface. This provides the possibility for high bandwidth utilization without a corresponding reduction in power efficiency. i.e., the possibility of high data rates within a limited bandwidth. This mechanism is known as spatial multiplexing. For example, considering a 2 by 2 antenna configuration, i.e., two transmit (Tx) antennas 14 and 16 and two receive (Rx) antennas 14′ and 16′ as shown in FIG. 6, and assuming that transmitted signals s1 and s2 are only subject to non-frequency-selective fading and white noise, the signal r (a bold symbol is understood to represent a vector) received at the receiver is described by:
      r    =                  (                                                            r                1                                                                                        r                2                                                    )            =                                                  (                                                                                          h                                              1                        ,                        1                                                                                                                        h                                              1                        ,                        2                                                                                                                                                        h                                              2                        ,                        1                                                                                                                        h                                              2                        ,                        2                                                                                                        )                        ·                          (                                                                                          s                      1                                                                                                                                  s                      2                                                                                  )                                +                      (                                                                                e                    1                                                                                                                    e                    2                                                                        )                          =                  Hs          +          e                      ,where H is the channel matrix and e represents the noise. In order for the receiver to be able to recover both signals s1 and s2, the H matrix should be invertible. Thus, the receiver calculates the inverse of H as exemplified by the following equation:
            (                                                                  s                ^                            1                                                                                          s                ^                            2                                          )        =                  W        ·        r            =                        (                                                                      s                  1                                                                                                      s                  2                                                              )                +                              H                          -              1                                ·          e                      ,where W is the inverse of H and ŝ is an estimate of the transmitted symbols at the receiver.
For systems in which a large amount of data is exchanged between transmitters and receivers, the receivers have to perform intense computational tasks for inverting a corresponding channel matrix H, which requires either expensive or bulky equipment at the receiver or a reduction in the exchanged data. Given the fact that the receiver is in some situations a mobile terminal, i.e., a cellular phone or other device capable of exchanging data with a base station, it is difficult to add computational power to the receiver without increasing the size and the cost of the receiver. Therefore, multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system but also require increased computational powers.
Accordingly, it would be desirable to provide methods, receivers and software for the receivers that would avoid the afore-described problems and drawbacks.